1. Field of the Invention
The present invention relates to a method for manufacturing metal nano particles. More particularly, the present invention relates to a method for manufacturing metal nano particles having a hollow structure and metal nano particles manufactured by the method.
2. Description of Related Art
Fuel cell is a power generating device obtaining the electric power by means of electrochemically reacting hydrogen gas with oxygen gas in the presence of a catalyst to generate water. The fuel cell is a product of a new power generating technology with high efficiency, low pollution, and diversified energy, and the hydrogen of the fuel cell enables the system to generate power, which not only has the advantages of cleanness and high efficiency, as compared with the conventional fossil fuel, but also it can further be combined with power generating technologies such as nuclear energy, biomass energy, solar energy, and wind energy, such that the usage of the energy is diversified, renewable, and continuous.
The fuel cell has a simple composition, and a modularized structure, which thus has a wide application scope, and the specific application field includes: space energy, life support system, submarine power, bus, car, locomotive, bicycle, distributed power generation, household independent power generation, commercial and industrial backup power generating system, PDA, notebook computer, cell phone, portable power supply for electrical products, and power unit for military/defense purpose.
In the fuel cell, an anode catalyst plays a crucial role in catalyzing the decomposition of the hydrogen gas to generate protons. After researching for several decades, the result shows that the platinum catalyst achieves the most preferred efficiency. In order to enlarge the active area of the reaction, and to reduce the using amount of the platinum, platinum is usually made into particles smaller than 5 nm. Since the size of the particles is reduced to the nanometer level, the platinum loses its original metal luster and presents a color of black, so it is called platinum black. As for the current technology, the electrolytic reaction of the hydrogen molecules can be effectively catalyzed when the using amount of the platinum in the catalyst is about 0.5 mg/cm2. Although the process for preparing the platinum black is simple, when the platinum black is used as the catalyst, the platinum black particles easily get close to each other and get aggregated, such that the active surface area is reduced, and the utilization efficiency of the catalyst is lowered.
In order to solve the above problems, recently two methods are proposed. The first method is using a protecting agent, dispersing agent, or a surface modifier to improve the dispersibility of the platinum blacks, but the improving efficiency is limited, and furthermore, the adopted protecting agent or the dispersing agent generates negative affects on the overall electron/proton conduction. The second method is using nanocarbon as the carrier (i.e. carbon-supported platinum catalyst) to effectively disperse the catalyst and to enhance the utilization efficiency. In addition, the carbon has a desirable electrical conductivity, and slightly affects the whole impedance. However, the size of the carbon capsules is relatively large (scores of nm), such that the thickness of the electrode layer is increased, and it is not easy for the fuel to diffuse into the electrode layer. Furthermore, the weather resistant characteristic of the carbon carrier is poor. Under a state of long-term discharging, it may be oxidized to carbon dioxide, which gradually escapes, and as a result, the electrode structure breaks down. The catalyst is the one with the highest cost among the materials for manufacturing electrodes, so that preparing a platinum catalyst with a higher effective surface area is quite important in enhancing the performance and reducing the cost.